154 SEWER DESIGN 



been deposited there may be scoured out from the bed of a 

 sewer or stream, or it may be held in suspension and so pre- 

 vented from accumulating deposits. To what laws or by 

 what means this power of water to hold material of greater 

 density in suspension is due is not clearly known. The sub- 

 ject, however, is of great importance, because if a sewer is to 

 be kept clean without intermittent hand labor, it must be 

 through the transporting power of the water which hurries 

 along with and in itself all solid matter. The admirable com- 

 pilation by Mr. E. H. Hooker on this subject, presented as a 

 thesis at Cornell University in 1896, and published later in 

 the Trans. Am. Soc. C. E., gives the following propositions, 

 applicable to sewers, as expressing the main facts so far as they 

 are known and necessarily underlying any broad theory of the 

 cause of the suspension of sediment: 



" i. The movements of solids by water may take place 

 by dragging, by intermittent suspension, or by continuous 

 suspension. 



" 2. Motion in each of the three ways is increased with 

 increase of depth; yet the depth itself can only affect the inter- 

 mittent suspension. 



" 3. Motion in each of the three ways is increased by 

 increase in the mean velocity. 



" 4. The presence of the sediment in the stream-flow 

 decreases its mean velocity. 



" 5. Dragging as well as suspending power increases with 

 heaviness of the liquid and with its greater coefficient of 

 viscosity. 



" 10. Increase of vortex motion increases the power of 

 transport. 



" 13. Bodies suspended in flowing water, either inter- 

 mittently or continuously, tend to acquire a velocity greater 

 than that of the water surrounding them." 



The theories offered to explain the facts or propositions 



